The present invention relates to a silane-crosslinking expandable polyolefin resin composition comprising a silane-grafted polyethylene resin obtained by melt mixing a linear low-density polyethylene, a vinyl-functional silane and a free radical generator, a polyolefin resin and an organic blowing agent, and to a crosslinked foam obtained by heating and foaming the composition.
Hitherto, various processes of producing a crosslinked foam using a polyolefin resin have been known. Chemical crosslinking, electron beam-irradiation crosslinking, moisture crosslinking (followed by condensation of silanols to siloxanes), and the like are widely well known as the method of crosslinking polyethylene. Of those methods, the moisture crosslinking does not require a large-scale crosslinking apparatus as compared with the chemical crosslinking and electron beam-irradiation crosslinking. Further, in the moisture crosslinking, a degree of crosslinking is determined according to the grafting amount of silane. Accordingly, the moisture crosslinking has an advantage that it is easy to control the crosslinking, as compared with the chemical crosslinking and electron beam-irradiation crosslinking. A method in which polyethylene is allowed to react and graft with a vinyl-functional silane in the presence of a free radical generator to undergo silane grafting is known as the method of preparation of silane-grafted polyethylene to be used for the moisture crosslinking, and a so-called silane crosslinking method in which the silane grafted polyethylene is brought into contact with moisture in the presence of a suitable condensation catalyst to undergo crosslinking is generally known (see, for example, JP-B-48-1711 and JP-A-57-49109).
However, although the moisture crosslinking is not expense in the apparatus cost as compared with other crosslinking methods, it involved a problem such that a long period of time and a high cost are required for the crosslinking treatment. For example, since in molded materials obtained from a moisture-crosslinkable resin composition, the progress of a crosslinking reaction is extremely slow at a temperature under the softening point of the composition, it is necessary to promote the crosslinking at a temperature above the softening point in high-humidity tank using hot water of about 80xc2x0 C. or steam. In order to completely proceed crosslinking reaction even by such a method, it takes about 24-36 hours, and hence, investment expenses of crosslinking treatment facilities are enormous. For this reason, even if the apparatus cost is not expensive, huge expenses for the crosslinking treatment are required.
In order to solve such problems, various attempts have hitherto been made in which a moisture-crosslinkable resin is subjected to extrusion molding, pipe molding, wire coating, or foaming, thereby shortening the crosslinking treatment time of the resulting product. For example, the following methods are proposed: a method in which a catalyst or an assistant is added to a silane-grafted polymer to promote the crosslinking (see, for example, JP-A-57-208006 and JP-A-62-106947); a method in which the contact with moisture during the crosslinking is enhanced to shorten the crosslinking time (see, for example, JP-A-60-254520); and a method in which the crosslinking treatment is carried out in a ultrasonic atmosphere to proceed with diffusion of water into the moisture-crosslinked resin, thereby shortening the crosslinking treatment time (see, for example, JP-A-4-331241).
However, although all of the above proposed methods can shorten the crosslinking treatment time by the hot water or steam treatment to some extent, in the case where the thickness of a molded material is large or in the case of a foam, a very long period of time was still required in order that the crosslinking uniformly proceeds into the central portion. In addition, in the foaming, since the crosslinking reaction is usually carried out prior to the foaming, the degree of crosslinking scatters between the vicinity of the surface and the central portion of the product leads to a cause of scatter in the cell size. For this reason, chemical crosslinking or electron beam-irradiation crosslinking is mainly employed for the crosslinking foaming using polyethylene.
The invention has been made to overcome the above-described problems in the prior art.
Accordingly, one object of the invention is to provide a silane-crosslinking expandable polyolefin resin composition that is flowable prior to the foaming and forms a crosslinked foam having superior heat resistance (heat deformation properties) after the foaming. In particular, the invention provides a silane-crosslinking expandable polyolefin resin composition that does not cause degassing even when foaming is carried out immediately after heat molding, and gives a crosslinked foam without need of the conventional crosslinking operation.
As a result of extensive and intensive investigations to overcome the above-described problems, it has been found that a silane-crosslinking expandable polyolefin composition comprising a mixture of a specified silane-grafted polyethylene resin and a polyolefin resin, and a specified organic blowing agent is flowable prior to foaming and forms a foam having superior heat resistance after the foaming. The invention has been completed based on this finding.
The invention provides a silane-crosslinking expandable polyolefin resin composition comprising:
100 parts by weight of a mixture comprising
5-90% by weight of a silane-grafted polyethylene resin obtained by melt mixing i) 100 parts by weight of a linear low-density polyethylene having a molecular weight distribution (Mw/Mn), as determined in terms of a converted value of linear polyethylene using 1,2,4-trichlorobenzene as a solvent by a gel permeation chromatograph, of 5 or less and a density, as determined according to JIS K7676, of 0.87-0.945 g/cm3, ii) 0.1-5 parts by weight of a vinyl-functional silane compound represented by the following formula:
RRxe2x80x2SiY2
wherein R represents a monovalent olefinic unsaturated hydrocarbon group, Y represents a hydrolyzable organic group, and Rxe2x80x2 represents a monovalent hydrocarbon group other than aliphatic unsaturated hydrocarbons or is identical with Y, and iii) 0.01-0.5 parts by weight of a free radical generator, and
95-10% by weight of a polyolefin resin, and
2-20 parts by weight of an organic blowing agent capable of generating an acidic substance and/or water as a decomposition product during heat decomposition.
The invention also provides a crosslinked foam obtained from the composition.